1. Field of the Invention
The present invention relates to a system for supplying secondary air into an exhaust system of an internal combustion engine and, more particularly, to a system for controlling the secondary air supply.
2. Description of the Prior Art
The heretofore known secondary air supply control system comprises a relief valve operative to maintain the pressure of the air in a secondary air supply line below a predetermined value and also comprises an air switching valve operative to direct the air from an air pump selectively to the engine exhaust system or back into an air cleaner in accordance with the engine operating conditions. The relief valve used therein is generally designed to be opened to release the pressurized air from the air supply line at a relatively low pressure level so as to increase the operative life of the air pump.
The percentages of HC and CO contents of engine exhaust gases, in general however, are increased when engine speed is abruptly accelerated or decelerated. The solution to the increase in the HC and CO emission at the engine acceleration relied substantially entirely upon the secondary air supply, although the solution to the HC emission increase at the engine deceleration relied upon other control systems. At the engine acceleration, therefore, it was required to supply the secondary air into the engine exhaust system as much as possible. However, because the relief valve in the secondary air supply line was opened to release the secondary air into atmosphere when the secondary air pressure exceeded the predetermined level, as discussed above, the engine exhaust system was not supplied with a sufficient amount of secondary air during engine acceleration operation despite the fact that the air pump was operated to discharge sufficient amount of secondary air.